The discovery of the unique catalysis afforded by the acetic acid solution of the joint use of one or more heavy metal oxidation catalysts and a source of bromide ion for the liquid phase oxidation at a temperature from 50 to 275.degree.C. of aliphatic-substituted aromatic compounds with molecular oxygen to aromatic polycarboxylic acid products was first disclosed in U.S. Pat. No. 2,833,816 which issued May 6, 1958. The use of said unique catalysis for such oxidation of xylenes under liquid phase conditions at 50.degree. to 275.degree.C. made feasible for the first time large scale commercial catalytic liquid phase production of the phthalic acids. Since 1958 many improved modes of conduct of such oxidations using the unique combination of heavy metal and bromide ion have been disclosed as advancements of that art. Some improvements were directed to yield improvement per unit of time and other improvements were directed to improved quality and yield of the phthalic acid products. In general the improved modes of conduct for said liquid phase oxidation using the unique catalysis involved use of either constant temperature or constant pressure; scheduling different rates of oxygen supply; use of sources of oxygen having oxygen contents below and above the oxygen content of air; regulation of water content of acetic acid solvent in the oxidation zone; sequentially staging of two or series connected oxidation zones operated at different temperatures, pressures, oxygen concentration or water concentration; use of different combinations of heavy metals and/or types of bromide iron source, e.g. ionic and combined bromine; and combinations thereof as applied to batchwise, semi-continuous and continuous operations. Such improved modes of operation using the unique catalysis did provide for yield increase of phthalic acid products from the yields demonstrated by the methods of U.S. Pat. No. 2,833,816. For example the yields of iso- and terephthalic acids from the corresponding isomeric xylenes were increased to 90-92 mole percent from 75-80 mole percent demonstrated by said patent with attendant improved decrease of partially oxidized xylene to such intermediates as aldehydobenzoic acid and toluic acid which contaminated said phthalic acid products.
Commercially feasible methods were devised for purifying the phthalic acid products from such improved modes of conduct of the aforementioned liquid phase oxidation using the unique catalysis. Such purifications were directed either to obtension of substantially colorless products for use in unsaturated polyesters or to obtension of iso- and terephthalic acid product of a purity of at least 99.9 weight percent for direct reaction with a diol for the preparation of polyesters of the high molecular weight required for film and fiber manufacture. However, little attention was given since 1958 to the introduction of a new component into the unique catalytic liquid phase oxidation to decrease partially oxidized xylene contaminants of phthalic acid products recovered from said oxidations.
It has been known since 1958 that oxidations of o-, m- or p-xylenes in the presence of the unique catalysis at temperatures in the range of 50.degree. to 120.degree.C. using oxygen gas as oxidant produced ortho-, iso- or terephthalic acid products containing relatively large amounts, 2 to 10 weight percent, of 2-, 3- or 4-formylbenzoic acid and like amounts of o-, m- or p-toluic acids. Oxidations under liquid phase conditions in the presence of the unique catalysis at temperatures above 120.degree.C., i.e., in the range of 120.degree. to 275.degree.C., using oxygen gas or air as source of molecular oxygen did decrease the contaminating formylbenzoic and toluic acid contaminants in ortho-, iso- and terephthalic acid products from the corresponding xylenes to below one weight percent, e.g. 0.5-1.0 weight percent. The improved modes of conduct of the catalytic liquid phase oxidation did little to improve low temperature (i.e., 50.degree.-100.degree.C.) oxidations but did increase phthalic acid product yields for the higher temperature oxidations.
To make more effective, on a pounds per hour throughput basis, the various commercially available purification routes for phthalic acids it is highly desirable to obtain such phthalic acids direct from oxidation of xylenes in a higher quality by some means in addition to such improved modes of conduct of liquid phase oxidation using the unique catalysis.